This invention relates to a printing method and a picture printing device employing such printing method. More particularly, it relates to a printing method in which a pixel is resolved into a matrix and the gradation of a picture is variably set by dots printed in each pixel, and a picture printing apparatus employing such printing method.
Video reproducing apparatus for reproducing picture signals recorded on a video tape, disc reproducing devices for reproducing picture signals recorded on a disc-shaped recording medium, such as an optical disc or a magneto-optical disc, and or picture reproducing devices for receiving and displaying television signals for outputting the received picture signals, such as a television receiver, are extremely popular in modern society. A picture printing device for printing a picture based on output picture signals of such picture reproducing devices has also been commercialized.
Picture printing devices may be classified into a heat-sensitive type device configured for heating a heat-sensitive sheet using a thermal head, a thermal transfer type in which a printing ribbon is heated using a thermal transfer head for transferring the printing ink from the printing ribbon to the printing sheet, and an ink jet system in which fine ink drops are deposited on the printing sheet.
A pixel 50 printed by the picture printing device is made up of a pre-set number of dots arranged in a 2.times.4 matrix having a main scanning direction X and an auxiliary scanning direction Y normal to the main scanning direction, as shown for example in FIG. 1.
The gradation of each pixel 50 forming the picture is variably set by printing a pre-set number of dots in each pixel depending upon pixel-based luminance of picture signals supplied from a picture signal input terminal.
With increase in luminance of picture signals, the gradation level of the pixel 50 is variably set by printing the respective dots as shown in FIG. 2 in a sequence indicated in FIG. 1. For gradation 0, no dots are printed, whereas, for gradations 1, 2 and 3, dots of the sequence numbers 1, 2 and 3 are printed, and so forth, until all dots are printed for the gradation 8. Thus, nine gradation levels from gradation 0 to gradation 8 may be displayed.
With the above picture printing device, printing is carried out by a printing head as dots are arranged line-by-line in the main scanning direction X, and the line number is incremented in the auxiliary scanning direction Y, for arraying pixels 50 in a matrix configuration on a printing sheet 60 for forming a picture, as shown in FIG. 3.
If printing is to be performed using a picture printing device of the heat-sensitive or thermal transfer system, in which a printing head is heated, the heat storage effect of the printing head is increased with an increase in the number of adjacent dots of a pixel during printing of these adjacent dots, the result is over printing the result over an area larger than the intended print area.
On the other hand, if printing is to be performed using a picture printing device in accordance with a heat transfer system or an ink jet system in which ink is deposited on a printing sheet, the effect of ink flowing and blending into adjacent ink drops is increased with an increase in the number of adjacent dots. Again, the result is printing over an area larger than the intended print area.
Thus an inconvenience arises in that an increase in the print density conflicts with sharply defined luminance of the picture signals. The result is that printing cannot be achieved with high print density and faithful reproduction of the gradations in the picture signals.